1. Field of the Invention
The present invention relates to a method for manufacturing a lower substrate of a liquid crystal display (LCD) device and, more particularly, to a method for manufacturing a lower substrate of an LCD device with high accuracy of alignment.
2. Description of Related Art
Currently, the LCD devices can be classified into two major categories, i.e., the passive matrix LCD (PMLCD) devices and the active matrix LCD (AMLCD) devices according to the driving method. The active matrix LCD device is a kind of display devices that utilize thin film transistors (TFT) as switching elements. Generally, the aperture ratio of a single pixel in the AMLCD device directly relates to the light quantity passing through transparent areas from back light modules. For the AMLCD devices having the same power consumption, the higher aperture ratio means the better brightness of AMLCD devices.
The designs of the high aperture ratios of the LCD devices are achieved by overlapping the periphery of the pixel electrode and the non-transparent conductive line, since the randomly aligned liquid crystal molecules form a light-leaking area between pixel electrodes could be shielded by non-transparent conductive signal line. However, because the alignment between the pixel electrode and the non-transparent conductive line is not accurate in practice, the resulting alignment shift usually causes the capacitance variance. Hence, the display quality is further deteriorated.
FIG. 1 (a) is a top view of the pixel area of the conventional LCD. In order to get high aperture radio, a transparent electrode 100 is arranged to partially overlap the conductive line. However, due to the limit of the resolution of pixel electrode masks, the alignment accuracy among layers, and the variation of side etching, the width of the conductive lines cannot be too narrow or otherwise the light leakage occurs. FIG. 1 (b) is a cross-section view of the line A-A′ in FIG. 1 (a). When the alignment between the transparent electrode layer 100 and the conductive line (i.e., the second metal layer 300) shifts, the shift will cause more variance in horizontal or vertical direction and result in inconsistent overlapping for layers. The inconsistent overlapping further causes various values of unexpected parasitic capacitance 400 and 401, and induces an unstable total parasitic capacitance. Thus the consistency of the display quality will be affected.